Process of polyesters

ABSTRACT

Method of retarding age embrittlement of polyester comprising heat treating the polyester after preparation by cooling below 90° C. and then heating at 90° C. to 160° C. within 24 hours of preparation, and a polyester composition in which ageing is substantially retarded compared to the non-heat treated polyester. The invention encompasses the polyester composition which has been subjected to the method.

This application claims benefit of international applicationPCT/GB94/01181, filed Jun. 1, 1994.

THIS INVENTION relates to processing of polyesters and in particular toproducing shaped articles less liable to embrittlement.

It has been disclosed by de Koning et al in Polymer 92, 33, (15),3295-3297 that whereas one such polyester poly(R)-3-hydroxybutyrate whenfreshly moulded shows ductile behaviour, within several weeks at roomtemperature the tensile modulus doubles and elongation at break dropsbelow 10%. A typical feature of such ageing is that it can be partlyreversed by heat or mechanical strain. Using mild `deageing` treatment,the improvement in ductility is only small and temporary, but heating atabove 100° C. restores the original mechanical properties and preventsor at least retards ageing to a large extent.

It has now been found that such age-embrittlement can be prevented orslowed by applying the heat treatment to shaped articles madesubstantially of polyhydroxybutyrate (PHB), polyhydroxybutyrateco-valerate (PHBV) and/or other polyhydroxyalkanoates before they haveundergone significant ageing.

Thus according to the invention there is provided a polyestercomposition comprising polyhydroxyalkanoate (PHA) in which ageing of thepolyester is retarded characterised in that after preparation thepolyester is cooled to below 90° C. and then heat treated at 90° C. to160° C. within 24 hours of preparation, and in that subsequent ageing issubstantially retarded compared to the non-heat treated polyester.

In a further aspect of the invention there is provided a shaped articleat least partly made of polyhydroxyalkanoate (PHA) in which ageing ofthe polyester is substantially retarded characterised in that afterpreparation the article is allowed to cool to below 90° C. is heattreated at 90° C. to 160° C. within 24 hours of preparation, and in thatsubsequent ageing is substantially retarded compared to the non-heattreated polyester.

"Ageing" means deterioration of the mechanical properties of polymerwith time, for example, ductility and tensile strength. Loss ofmechanical properties can be measured by measurements indicative ofageing, for example, elongation to break and impact resistance (Izod).

In the present context "substantially retarded" means that for practicalpurposes the polyester or article is not defective for brittleness orductility for at least one month after the heat treatment. Preferablythe polyester or article shows at least 40%, especially 60%, improvementin brittleness or ductility compared to the non-heat treated polyesteror article. This can be measured using any measurement indicative ofageing, for example, by elongation to break measurements or impactstrength measurements (Izod).

By "within 24 hours" is meant up to 24 hours, preferably within 12 hoursof preparation, particularly 60 minutes or less from preparation.

"At least partly made" means having structural components made of PHA,for example PHB or PHBV, to such an extent that embrittlement of the PHAcomponents embrittles the whole article. Thus for example articleshaving PHB components linked to other components, and articles made of amatrix of some other polymer with PHB inclusions, are within theinvention. Articles made of PHB alone benefit most from the invention.

The heat treatment can be any that does not involve a step effectingsubstantial crystallisation of the polyester in the article presented tothe heat treatment at 100° C. to 140° C.

If shaping has been by melting or sintering, the heat treatment mayfollow immediately upon shaping, for example by holding an article inits mould for a suitable time; the mould temperature may be deliberatelyadjusted or may be arrived at as the resultant of the molten polymertemperature and the initial mould temperature. If desired, the articlemay be held at a temperature appropriate for crystallisation before theheat treatment according to the invention. If shaping has beencontinuous, for example by extrusion of fibre or film or a profile, theheat treatment can be effected in a zone downstream of the extruder andany quench, crystallisation or draw zone.

The PHA is especially capable of a relatively high level ofcrystallinity, for example over 30%, especially 50-90%, in the absenceof plasticiser. Suitably it is or includes at least onemicrobiologically produced polyester having units of formula I:

    --O--C.sub.m H.sub.n --CO--                                I

where m is in the range 1-13 and n is 2m or (if m is at least 2) 2m-2.Typically C_(m) H_(n), contains 2-5 carbon atoms in the polymer chainand the remainder (if any) in a side chain. In very suitable polyestersm is 3 or 4, n is 2m and especially there are units with m=3 and m=4copolymerised together and with respectively a C₁ and C₂ side chain onthe carbon next to oxygen in the chain. Particular polyesters contain apreponderance of m=3 units, especially homopolymer (PHB) or with atleast 70 mol %, preferably 70-98% of such units, the balance (if any)being units in which m=4 (PHBV). The molecular weight of the PHA ispreferably over 50000, especially over 100000, up to eg 2×10⁶.

The PHA is conveniently a blend of two or more polymers or copolymersdiffering in the value of m. A particular example contains

(a) PHA consisting essentially of Formula I units in which 2-5 mol % ofunits have m=4, the rest m=3; and

(b) PHA consisting essentially of Formula I units in which 5-30 mol % ofunits have m=4, the rest m=3.

In each such PHA there are side chains as above mentioned. Theproportions in such a blend are preferably such as give an average m=4content in the range 4-20 mol %.

In each such PHA having units with m=3 and/or m=4 there may be verysmall, typically fractional, percentages of units having higher valuesof m.

Typically the composition contains microbiologically produced PHA to theextent of over 50% w/w, especially over 80% w/w.

PHA comprising hydroxybutyrate units and hydroxyvalerate units includesPHBV copolymers containing up to 1 mol percent of other oxyalkanoateunits whether introduced deliberately or not.

The polyhydroxyalkanoate is preferably polyhydroxy-butyrate (PHB) orpolyhydroxybutyrate-co-valerate (PHBV), which may be 3-hydroxy or4-hydroxy or a mixture of both. Especially preferred are the(R)-3-hydroxy forms of PHB and PHBV.

The PHA is preferably a fermentation product, especially of amicrobiological process in which a microorganism lays down PHA duringnormal growth or is caused to do so by cultivation in the absence of oneor more nutrients necessary for cell multiplication. The microorganismmay be wild or mutated or may have had the necessary genetic materialintroduced into it. Alternatively the necessary genetic material may beharboured by a eukaryote, to effect the microbiological process.

Examples of suitable microbiological processes are the following:

for Formula I material with m=3 or m=partly 3, partly 4: in EP-A-69497(Alcaligenes eutrophus);

for Formula I material with m=3: U.S. Pat. No. 4,101,533 (A. eutrophusH-16), EP-A-144017 (A. latus);

for Formula I material with m=7-13: EP-A-0392687 (various Pseudomonas).

The PHA can be extracted from the fermentation product cells by means ofan organic solvent, or the cellular protein material may be decomposedleaving microscopic granules of PHA. For specialised end uses thecellular protein may be partly or wholly allowed to remain with the PHA,but preferably subjected to cell breakage.

Alternately if the PHA can be a product of synthetic chemistry usingprocesses known in the art. PHB can be prepared according toBloembergen, S. and Holden, D. A., Macromolecules. 1989, Vol 22,p1656-1663). PHBV can be prepared according to Bloembergen, Holden,Bluhm, Hamer and Marchessault, Macromolecules. 1989, Vol 22, p1663-1669.

The properties of the polyester or article of the present invention canbe assessed using the following measurements : stress-strain curveincluding calculations of elongation to break, Youngs modulus, andtensile strength; impact testing, for example Izod; and dynamicmechanical thermal analysis (DMTA). These are all standard methods fortesting mechanical properties.

If the PHA contains m=3 units and then 1, especially more than 5, molpercent of units in which m=4 or greater than 6, the compositionsubjected to shaping preferably contains a plasticiser.

The polyester composition can contain the usual polymer processingadditives such as plasticisers, particulate or fibrous or platy filleror reinforcer, fibres, nucleating agents (for example boron nitride,talc or ammonium chloride), and pigments. The nucleant is preferablypresent in 0.1 to 10 phr (parts per hundred), especially 1 to 5 phr. Thecomposition can be in the form of mouldings, extrudates, coatings, filmsor fibres, including multilayer coatings, films or fibres.

The plasticiser is any material capable of plasticising polyester, i.e.capable of improving the ductility of the polyester and especially anymaterial capable of plasticising PHB or PHBV. There may be one or moreplasticisers present. For the ratio of plasticiser to PHA the range upto and including 40 phr w/w, preferably 1 to 40 phr w/w includes most ofthe likely uses and for making effectively rigid but not brittlearticles the range 5-20 especially 6-12, phr w/w is generally suitable.

Examples of suitable plasticisers are

(a) high-boiling esters of polybasic acids, such as phthalates,isophthalates, citrates, fumarates, glutamate, phosphates or phosphites.The esterified radicals may be for example C₁ -C₁₂ alkyl, aryl, aralkylor aralkyl. Particular examples are dioctyl-, dibeptyl- and dirindecyl-phthalates and dialkylalkylene oxide glutamate (Plasthall 7050);

(b) high boiling esters and part- of polyhydric alcohols, especiallyglycols, polyglycols and glycerol. The acid-derived radical of the estertypically contains 2-10 carbon atoms. Examples are triacetin, diacetinand glyceryl dibenzoate;

(c) aromatic sulphonamides such as paratoluene sulphonamide.

A particularly preferred plasticiser is a doubly esterifiedhydroxycarboxylic acid having at least 3 ester groups in its molecule.

"Doubly esterified" means that at least some of the hydroxy groups ofthe hydroxycarboxylic acid are esterified with a carboxylic acid and atleast some of the carboxy groups thereof are esterified with an alcoholor phenol. Preferably at least the hydroxycarboxylic acid from which theester is derived is aliphatic or cycloaliphatic. Its backbone structure(that is, apart from carboxy groups) preferably contains 2-6 carbonatoms. It contains preferably 2-4 carboxy groups and 1-3 hydroxy groups;and preferably the number of carboxy groups exceeds the number ofhydroxy groups.

The groups with which the carboxy groups are esterified containpreferably 1-7, especially 2-5 carbon atoms. In the ester molecule theycan be the same or different. Preferably they are aliphatic. For thermalstability but biodegradability such aliphatic groups preferably havestraight chains. If desired, a small portion of these groups aredivalent, so as to give an oligomer suitably containing up to 3repeating units.

The groups with which the hydroxy groups are esterified preferablycontain 2-7, especially up to 4, carbon atoms, including the carbon atomof the carboxy of such groups. In the ester molecule such groups can bethe same or different. Preferably they are aliphatic and, for thermalstability and biodegradability, have straight chains. If desired, asmall proportion of these groups are divalent, so as to give an oligomersuitably containing up to 3 repeating units. An example of such aplasticiser is Estaflex*ATC.*indicates a tradename or registeredtrademark.

This list is not exhaustive and any plasticiser of polyester which isnot listed above or which becomes available after the date of thisapplication would be suitable for use in this invention.

The process used to shape the article can be for example any one ofinjection moulding, compression moulding, extrusion of fibre or films,extrusion of profile, gas-current spinning, tack spinning, coating meltonto substrate, coating latex onto substrate, shaping solution involatile solvent.

The invention also provides a method of retarding age embrittlement ofpolyester comprising heat treating said polyester characterised in thatafter preparation the polyester is cooled to below 90° C. and then heattreated at 90° C. to 160° C. within 24 hours of preparation.

The heat treatment is more preferably in the range 90° to 150° C.,especially 100° C. to 140° C.

The heating time is typically at least a few seconds, preferably from 5seconds to 20 hours, especially 0.5 minutes to 14 hours, particularly0.5 minutes to 2 hours.

The heating time required for optimal effect is dependent on the heatingtemperature, i.e. the higher the temperature the less time that isrequired to achieve optimal effect. Also the temperature can be chosento suit the characteristics of the processing plant and economicrequirements.

Heating can be effected in air or oxygen-depleted or inert gas or invacuo, or in water or a fluid which does not interfere with theintegrity of the polyester, or in a mould. Heat transfer can be byconduction, radiation, convection or resistive heating. Heat transfermethods may include ovens, water baths and hot rollers. A preferred formof heat transfer is by infra red radiation, for example, black body andquartz tubes. The shaped article is generally subjected to infra redradiation for 30 seconds to 15 minutes, preferably 30 seconds to 10minutes.

The shaped articles may be run through the oven or other heating methodon a continuous belt at a speed which is optimal to enable the shapedarticle to reach the correct temperature. A preferred method is to havea multi-zone system, preferably a 2 zone heat system in which the firstzone gives a rapid rate of heating (i.e. the actual temperature in thezone may higher than that to be achieved by the shaped article) to bringit to the actual temperature required and then in the second zone theshaped article is maintained at the actual temperature to be achievedfor the desired time period.

The invention provides methods of making the composition by mixing itscomponents. If desired, this may be effected in a solvent, such as ahalogenated hydrocarbon or alkylene carbonate. Such a method isconvenient for coating or for centrifugal spinning of fibres. Moreconveniently, if a plasticiser is present, the plasticiser is mixed withpowdered dry polymer and the mixture is treated in conditions of shear,such as in a plastic mill or extruder. The product is then granulatedand used as feed for a shaping operation such as extrusion, injectionmoulding, injection blow-moulding or compression moulding.

The composition is especially useful for making the following shapedarticles: films, especially for packaging, fibres, non-woven fabrics,extruded nets, personal hygiene products, bottles and drinking vessels,agricultural and horticultural films and vessels, ostomy bags, coatedproducts (such as paper, paperboard, non-woven fabrics), agriculturaland horticultural films and vessels, slow-release devices.Alternatively, the polymer composition with suitable additives can beused as an adhesive.

The invention is now further described, but is not limited by, thefollowing examples.

FORMULATION AND TEST PROCEDURES

In the following examples, the PHB and PHBV polymers were theR!-3-hydroxy form. Compositions were prepared by mixing copolymer (500g) with 1 phr boron nitride and (if required) plasticiser, and extrudingthe mixture in a Betol 2520 extruder in these conditions:

    ______________________________________                                               Zone 1         130° C.                                                 Zone 2         140° C.                                                 Zone 3         150° C.                                                 Die            150° C.                                                 Screw Speed    100 rpm                                                 ______________________________________                                    

The extrudate, a single 4 mm lace, was crystallised at 50°-60° C. in awater bath, dried in a current of air and cut into granules.

Granules were then injection moulded (Boy 15 S) into hot moulds, to givedumbbell-shaped tensile bars according to ISO R 537/2, their prismaticpart measuring 40×5×2 mm. The mould temperature was held at 60° C. afterentry of the polymer. The bars were numbered as they came out of themould, then allowed to cool at ambient temperature.

Injection moulding conditions were:

    ______________________________________                                        Barrel Zone 1          130° C.                                         Barrel Zone 2          130° C.                                         Nozzle                 130° C.                                         Mould heater temperature                                                                             74-77° C.                                       Mould temperature      60° C.                                          Pressure hold on time  12 sec                                                 Cooling time           30 sec                                                 Injection pressure     45 bar                                                 Screw Feed             250 rpm                                                ______________________________________                                    

After removal from the mould the bars were stored for various periods atambient temperature and then subjected to tensile testing using anInstrom 1122 fitted with a Nene data analysis system. The jaw separationused was 50 mm and crosshead speed was 10 mm/min⁻¹.

EXAMPLE 1

The composition PHB 10 V and 1 phr boron nitride+10 phr dioctylphthalate(Jayflex DIOP) (RTM) was prepared and moulded as described. The barswere removed from the mould. The bars were then heat treated at 110° C.for 1 hour. The bars were tested for elongation to break over variousperiods. The lower the elongation to break the less ductile the polymer.Results are shown in Table 1 as percentage elongation to break.

                  TABLE 1                                                         ______________________________________                                        Ageing time after heat treatment(days)                                        0             1      3         7    28                                        ______________________________________                                        Control 459.3     15.3   --      13.8 12.8                                    treated 282.5     --     44.8    30.6 29.0                                    ______________________________________                                         (--) means not tested.                                                   

It is evident that for the heat treated polymer the extension to break,although initially considerably less than for the non-heat treatedpolymer, decreases less rapidly over a period of time to a levelsignificantly (over 100%) and usefully higher than that of the untreatedpolymer.

EXAMPLE 2

The experiment of Example 1 was repeated using plasticised homopolymer(1 phr BN as nucleant and 20 phr Diolpate PPA 350 as plasticiser).

The bars were released directly after moulding. Within 30 seconds of demoulding the bars were heat treated at 110° C. for 1 minute. The barswere tested for extension to break and the values are represented aspercentage extension to break.

                  TABLE 2                                                         ______________________________________                                               Ageing time after treatment (days)                                            0    1        7      28     84   185                                   ______________________________________                                        Control  619    340      21.2 17.4   14.9 11.95                               treated  540    427      103  34.9   25.8 21.9                                ______________________________________                                    

It is evident that the extension to break for the heat treated polymer,although starting from a lower value, does not decrease as rapidly asthe non-heat treated polymer and is still significantly higher(approximately 100%) than the corresponding value for the non-heattreated polymer at 6 months after treatment.

EXAMPLE 3

The experiment of Example 1 was repeated using plasticised homopolymer(1 phr BN as nucleant and 20 phr Reoplas 39 as plasticiser).

The bars were released directly after moulding. Within 30 minutes aftermoulding the bars were heat treated at 140° C. for 30 minutes. The barswere tested for extension to break and the values are represented aspercentage extension to break.

                  TABLE 3                                                         ______________________________________                                        Ageing time after heat treatment(days)                                        0             1      3         7    28                                        ______________________________________                                        Control 55.2      20.5   10.5    7.4  6.3                                     Treated 49.7      42     42.9    30.2 --                                      ______________________________________                                    

These results also demonstrate that at 28 days after treatment the heattreated polymer is significantly more ductile than the non-heat treatedpolymer.

We claim:
 1. In a process of producing a shaped article at least partlymade of a polyhydroxyalkanoate selected from the group consisting ofpoly-(R)-3-hydroxybutyrate andpoly-(R)-3-hydroxybutyrate-co-(R)-valerate by the steps of:(a) shapingby injection molding, compression molding, melting, sintering, extrusionof fiber or film or profile, gas-current spinning, tack spinning,coating of melt onto substrate, coating latex onto substrate, or shapingsolution in volatile solvent; (b) allowing the shaped article to coolbelow 90 deg C.; and (c) heat treating at 90-160 deg C. withoutsubstantial change of shape; the improvement wherein step (c) isperformed within 24 hours of the end of step (a), whereby the resultingshaped article is characterized by its resistance to brittleness onaging.
 2. Process according to claim 1 in which the PHA containsplasticiser.
 3. Process according to claim 1 in which the temperature ofstep (c) is in the range 100°-140° C.
 4. Process according to claim 1 inwhich the PHA contains 4-20 mol % of valerate.
 5. Process according toclaim 1 in which step (c) takes 5 sec to 20 h.
 6. Process according toclaim 1 in which steps (a), (b) and (c) are carried out in immediatesuccession, the articles formed in step (a) being run through heatingzones on a continuous belt and heat being supplied to step (c) by infrared radiation for a period in the range 30 seconds to 15 minutes.
 7. Ashaped article made by the process of claim 1 and being at least partlymade of poly-(R)-3-hydroxybutyrate-co-(R)-valerate copolymer containing70-98 mol % of such butyrate units, characterised by not being defectivefor brittleness for at least one month after being formed.
 8. In aprocess of producing a shaped article at least partly made ofpoly-(R)-3-hydroxybutyrate-co-(R)-valerate by the steps of:(a) shapingby injection molding, compression molding, melting, sintering, extrusionof fibre or film or profile, gas-current spinning, tack spinning,coating of melt onto substrate, coating latex onto substrate, or shapingsolution in volatile solvent; (b) allowing to cool below 90° C.; and (c)heat treating at 90°-160° C. without substantial change of shape; theimprovement which comprises performing step (c) within 24 hours of theend of step (a) whereby the resulting shaped article is characterised byits resistance to brittleness on ageing.